1. Field of the Invention
The present invention relates to a latent electrostatic image bearing member for use in image formation processes employing electrophotography, such as in copiers, electrostatic printing, printers, facsimiles and electrostatic recording, and to an image forming apparatus, image forming method and process cartridge using the latent electrostatic image bearing member.
2. Description of the Related Art
In an image forming apparatus utilizing electrophotography such as a copier, printer or facsimile machine, a latent electrostatic image bearing member (hereinafter referred to as “photoconductor,” “electrophotographic photoconductor,” or “image bearing member” in some cases) that is uniformly charged is irradiated with a write beam modulated by image data to form thereon a latent electrostatic image, and a toner is supplied to the image by means of a developing unit. Thereafter, the developed image (toner image) on the photoconductor is transferred onto a recording medium (recording paper), and heated and pressed against the recording medium by means of a fixing unit so as to be fixed onto the recording medium. Meanwhile, toner particles that remain on the photoconductor surface are scraped off by means of a cleaning blade for recovery.
In such image forming apparatus employing electrophotography, organic photoconductors are widely used that contain organic optical photoconductive material. Organic photoconductors are advantageous over other conductive photoconductors in such aspects as easiness with which materials that lend themselves to various exposure beam sources ranging in wavelength from visible to infrared beams are developed, selectivity for materials that have little environmental impact, and lower manufacturing costs. These organic photoconductors, however, have a problem that they offer low mechanical strength and thus its organic photoconductor layer wears off during a long time use, and therefore, when the photoconductor layer has worn off to a given degree, it results in change in the photoconductor's electrical characteristics and in failure to conduct proper image formation process. The photoconductor layer wears off at any position where it contacts other image formation units such as a developing unit and a transfer unit during image formation process.
To overcome this problem, studies have been made to increase the photoconductor life by reducing wear, and various suggestions have been presented in order to achieve this. For example, Japanese Patent Application Laid-Open (JP-A) No. 06-118681 suggests employing a colloidal silica-containing curable silicone resin as a surface protective layer for photoconductor.
JP-A Nos. 09-124943 and 09-190004 both suggest a photoconductor having on its surface a resin layer in which an organic silicon-modified hole transporting compound is bound to curable organic silicon-based polymer molecules.
JP-A No. 2000-171990 suggests a method of curing curable siloxane resin with charge transportability-imparting groups such that the cured resin has a three-dimensional network.
JP-A No. 2003-186223 suggests a photoconductor having a protective layer that contains therein a charge transporting substance having at least one hydroxyl group, three-dimensionally crosslinked resin, and conductive particles.
Although the invention disclosed in JP-A No. 06-118681 succeeded in improving the wear resistance of the surface layer made of colloidal silica-containing curable silicone resin, the photoconductor offers poor electrical characteristics after repetitive use, fogging and/or image blur are more likely to occur, and durability is smaller than those of longer-lasting photoconductors that have been required in recent years.
With the inventions described in JP-A Nos. 09-124943 and 09-190004, the photoconductor's resin layer tend to cause image blur and hence a drum heater or the like is required to prevent this before put into practical use, resulting in increased apparatus size and costs. Moreover, inability to sufficiently reduce the residual potential of the exposed area may lead to reduced image density in a low-potential development process where image formation is effected with reduced charging potential.
With the invention disclosed in JP-A No. 2000-171990, it may result in the generation of cracks in the coating, which are considered to be caused by volume contraction of the coating. This problem occur particularly where a cheap, easy-to-handle commercially available coating agent is used in combination. In addition, the level of residual potential of the exposed area is dependent on the coating thickness, and reduced image density in low-potential development process may become a problem. Moreover, an increased content of charge transportability-imparting groups leads to reduced coating strength, and as such provision of sufficient durability may fail. The increased content of charge transportability-imparting groups may further cause image blur. Accordingly, it is difficult to obtain an electrophotographic photoconductor easily and inexpensively which is capable of output of excellent images over a long period of time.
With the configuration of the invention disclosed in JP-A No. 2003-186223, it may succeed in achieving increased wear resistance and reduced residual potential to some extent; however, simply adding conductive particles in the protective layer decreases the volume resistance of the protective layer. For this reason, image blur tends to occur due to latent electrostatic image displacement under high-temperature, high-humidity conditions. In addition, since the charge transporting substance also constitutes the structural unit of the three-dimensionally crosslinked network of the protective layer, the greater the proportion of the charge transporting substance in the protective layer, the greater the influence of its molecular structure—particularly the number of hydroxyl groups and/or the binding sites—on the wear resistance. Thus, in some instances, it may result in failure to obtain sufficient wear resistance.
As a result of the extensive studies conducted by the present inventors, it is becoming increasingly established that an electrophotographic photoconductor, of which surface layer is a charge transporting urethane resin layer prepared by polymerizing an isocyanate compound and a reactive charge transporting substance having a specific structure, offers significantly increased wear resistance while achieving reduced residual potential. It is, however, also established that electrophotographic photoconductors having such a configuration cause image blur due to latent electrostatic image displacement when their surface layer is exposed to oxidizing gas such as ozone or NOx. In a digital image forming apparatus, such image blur reduces the image density of halftone images formed of fine dots, leading to abnormal images. Thus, there still remains a need in the art to achieve further improvements.